The present invention relates to additives of the aminopolyamide-epichlorohydrin (PAE) type used to increase the wet strength of paper, and more precisely to an improvement for limiting its impact on the environment.
The paper has natural mechanical strength due to the interlocking of millions of xe2x80x9cwoodxe2x80x9d fibers. This dry strength is due to the numerous hydrogen bonds established during the drying of the paper. During the water evaporation process, the acid hydrogens of the fibers attract one another so as to form hydrogen bonds. In the presence of water, the mechanical strength of the sheet of paper deteriorates rapidly as the interfibrous hydrogen bonds disappear: water will progressively separate the fibrous lattice by weakening the fibers themselves.
In order to consolidate the fibrous mat and to partially prevent the water from reaching the hydroxyl groups of the fibers, chemists have proposed improving the strength of the paper by, among other things, adding thermosetting resins, more specifically thermosetting resins with cationic characteristics to facilitate their adsorption into the fibers, which have certain anionic characteristics. Two principal systems have been developed and have competed on the market: the first uses aminoplast(urea-formaldehyde or melamine-formaldehyde) resins, and the second system, developed in the 1960s uses a polyamide-polyamine-epichlorohydrin (PAE) resin in a neutral medium. The latter process, which has expanded since its creation, has numerous advantages over the aminoplast resins used in an acid medium (pH 4.5-5), for example the fact that it can use less expensive fillers such as calcium carbonate, and that it causes less corrosion of industrial equipment. These resins have another advantage over the aminoplast resins in that they do not include formaldehyde, which is considered to be a noxious element. These PAE resins, due to the fact that the process for obtaining them uses epichlorohydrin (EPC), contain organochloric products, and because of this they are subject to regulation which severely limits the emission into the environment of organic halogens which are adsorbable by activated carbons. These products, designated AOX (for xe2x80x9cadsorbable organic halogensxe2x80x9d), are known to be measurable in water in accordance with various standards (DIN 38409, SCAN-W9:89, ISO 9562).
PAE resins are manufactured industrially in two steps. The first step combines a dicarboxylic acid, adiphic acid, and diethylenetriamine (DETA), which are condensed at a high temperature. The polyamide polyamine resin (PA) formed in this way reacts with the epichlorohydrin to form the PAE resin, which is a polymer of low molecular weight which can be cross-linked, and which is stabilized in an acid medium.
The reactions for obtaining the polyamide-polyamine-epichlorohydrin resin (PAE) are summarized in the FIGURE. The epichlorohydrin reacts with the secondary amines, and probably also the primary amines, by combining azetidinium groups on one side and adding chlorohydroxypropane groups on the other side, the PAE resin being a oligomer which contains, in a random distribution, azetidinium groups of this type with inorganic chlorine, and lateral groups with organic chlorine. The reaction which consists of the alkylation of the PA by the epichlorohydrin is incomplete. After the reaction, several hundred ppm of free epichlorohydrin remains, part of which is hydrolyzed in an aqueous medium, chiefly 1,3-dichloro-2-propanol (DCP) and also 3-monochlore-1,2propanediol (MCPD), products of low molecular weight. These products are AOXs. DPC is recognized to be toxic and mutagenic. MCPD is also recognized to be toxic and is feared to be mutagenic. Their presence in products used for treating paper and in the effluent from paper manufacturing thus constitutes a troubling problem. Their measurement is easily achieved by means of gas chromatography in a Superwax(copyright) column with a polyethylene glycol phase with flame ionization detection.
In order to solve the AOX problem, the principal PAE resin suppliers have developed new resins with reduced AOX and organic chlorine contents (for example Bayer""s U.S. Pat. No. 4,857,586 or European patent 540,943). The use of synthesis to reduce the DCP content to about 550 ppm is well known. As a result of this, however, the resin progressively loses its ability to function as a wet strength additive for the paper. So these resins, which are reputed to have a low 1,3-dichloro-2-propanol content, nevertheless contain quantities of it which are non-negligible, on the order of several hundred ppm, some of which will remain in the paper and some of which will pass into the white water of paper mills. All of these organochlorines can be sources of contamination for the environment (see Devore, David I., Clungeon, Nancy S., Fischer, Stephen A., Tappi Journal 74, 12, 1991, 135-141, Henkel Corp.). One solution which seems to present itself is the retreatment of the resin through adsorbents in order to extract these undesirable chloric compounds from it. International patent application WO 92/22.601 therefore proposes a retreatment through ion exchange resins; but here again, the resin, purified of DCP, perceptibly loses its wet strength qualities.
International patent application WO 93/21.384 (E.I. Du Pont de Nemours and Co.) discloses the possibility of reducing the secondary products resulting from the hydrolysis of the epichlorohydrin in a single percolation through a bed of adsorbent, and more precisely through a bed of ion exchange resins.
As a solution to this unsolved problem, Applicants propose a very effective and economical process, which is the object of the present invention, for producing a PAE resin which contains only traces of the toxic organochloric products resulting from the degradation of the epichlorohydrin, which are ordinarily undetectable, by selectively trapping the DPC, the MCPD and the EPC present in the PAE resins in activated carbon.
In accordance with the present invention there is provided a process for lowering EPC, DCP and MCPD content of PAE resins comprising passing aqueous polyamide-polyamine-epichlorohydrin (PAE) resins with a dry extract between 2 and 60% through a first activated carbon bed until 1,3-dichloro-2-propanol (DCP) appears in the percolate, then through a second activated carbon bed until 3-monochloro-1,2-propanediol (MCPD) appears in the percolate.
In accordance with another aspect of the present invention, there is provided a composition for treating paper comprising an aqueous polyamide-polyamine-epichlorohydrin resin having a dry extract between 2 and 60%, a 1,3-dichloro-2-propanol content relative to its dry extract that is not measurable, a 3-monochloro-1,2-propanediol content less than 250 ppm and an organic chlorine content, also relative to its dry extract, that no less than 1%.